1. Technical Field
The invention relates to the monitoring of the pressure of a product fluid to be administered in dosed amounts during infusion or injection.
2. Description of the Related Art
During infusion or injection of a product fluid, the exact dosing of the administered product is of large importance, especially upon dosed administration to humans. If the administration is largely automated, as, for example, in insulin treatment with portable infusion pumps, monitoring of the proper product administration is desired, if not a must. A significant possibility for the monitoring is the monitoring of the product fluid pressure, since during proper functioning of the mechanical and electronic components of an infusion or injection apparatus, a proper product administration can be inferred as long as the fluid pressure remains within a preselected pressure range. Upon noncompliance, an occlusion or leakage in the fluid guide system can be inferred. Leakages and occlusions present a risk, especially during the automated administration of active substance solutions in the medical or veterinary field because of the danger of non-detection or detection too late.
Infusion pumps wherein the pressure of the product fluid to be administered is monitored in order to detect blockages in the fluid guiding system are known, for example, from U.S. Pat. No. 4,562,751, and WO 96/27398. These two references relate to infusion pumps wherein a product fluid is conveyed from a container containing the product by displacement by means of a piston. The piston is linearly displaceable in the container and is driven by a spindle drive positioned parallel to a displacement axis of the piston. A spindle drive element is mounted in the housing of the infusion pump for rotation around its longitudinal axis and supported for displacement in direction of its longitudinal axis. A nut which acts as a driven member and moves on the spindle is secured in the housing against rotation and coupled with the piston in such a way that it moves the piston along during its own linear displacement.
With its free end on one end surface, the spindle according to U.S. Pat. No. 4,562,751 engages a thrust switch which upon exceeding of a threshold pressure stops the pump. This threshold pressure is reached when the piston is not moveable in the container or only with excessively large force. In such a case, the spindle because of its rotation in the nut, moves opposite the displacement direction of the piston and against the thrust switch.
It is mentioned in WO 96/27398 as a disadvantage of such type of pressure monitoring that upon use of such a high pressure limit switch, an occlusion is not indicated in time, since, first, several product dosages, which should have been administered in series, must so to speak accumulate in order to build up the threshold pressure for the switch. The WO 96/27398 suggests the use of a force sensor instead of the use of a high pressure limit switch, which force sensor outputs a signal which is proportional to the measured force and, thus, the fluid pressure in the container. The pressure monitoring consists of detecting the fluid pressure by the force sensor at two different points in time by measuring of the reaction force exerted by the piston during its advance in the container, and comparing the two signals output by the force sensor with one another. Therein a first measurement takes place during administration of the product fluid and a second measurement later before administration of the subsequent product dose. If a preselected difference between the two signals is not reached, an occlusion is indicated and an alarm signal activated. If the difference exceeds the preselected signal difference, this then indicates that the pressure in the container has fallen, and that product fluid is in fact administered in the required manner. This type of pressure monitoring takes substantial time. Furthermore, only occlusions are detectable.
It is an object of the invention to enable a pressure monitoring of a product fluid to be administered in dosed amounts during infusion or injection wherein malfunctions are detected as quickly and reliably as possible.
The invention is based on a monitoring of the pressure of a product fluid to be administered in dosed amounts during infusion or injection wherein the product fluid is dispensable or dispensed from a container by advancing a piston received in the container. The product fluid is understood to be especially a liquid solution of an active substance. The container is received in a housing or formed by the housing itself. As measure of the fluid pressure, a reaction force is measured which is exerted by the piston during advancement and is fed to a control for a drive of the piston. The control compares this measured reaction force with a preselected reference force and controls the drive of the piston in accordance with the comparative results resulting therefrom.
In accordance with the invention, the reference force is a nominal value for the reaction force and a direct nominal/actual comparison is carried out between the measured reaction force and its nominal value. This at least one nominal value is stored and can always be recalled by the control for the purpose of the nominal/actual comparison for comparing it with the actual reaction force measured during a dispensing operation, especially an administration or priming. The reaction time in case of a malfunction determined during the monitoring can be held as short as possible by the control making the direct comparison between the actually measured reaction force and its nominal value, i.e., measuring the difference between nominal and actual.
Preferably, the control only monitors or controls the actual reaction force and only in the case where a preselected maximum admissible difference between nominal and actual is exceeded reacts with the shutdown of the drive and appropriately also with the activation of an alarm. Apart from this consideration in accordance with the invention, the control as part of the drive control can also start a program as is done by conventional controls.
In an especially preferred embodiment, a valve is positioned in the path of the product fluid between the outlet of the container and the exit location, for example, the forward end of an infusion needle, said valve causing a predefined pressure drop. The pressure drop is preferably selected so that a leaking of product fluid from the container because of gravity is safely prevented during all conditions occurring during the practical use of the device. For the construction and positioning of such a valve, reference is made to German Patent Application No. 197 23 648, the disclosure of which is herewith incorporated. The valve is preferably positioned as closely as possible to the exit location of the product fluid in order to include the whole fluid conducting system into the leakage monitoring. If the valve is positioned in the flow path of the product fluid immediately after the container outlet, which corresponds to another preferred embodiment, leakage between the piston and the valve can still be detected.
Additionally acting forces can be neglected compared to the valve action. That reaction force which results based on the nominal value for the pressure drop of valves of a valve model series is used as the nominal value. The control is adjusted to this nominal value by the manufacturer. The permissible range for the actual reaction force is preselected such that it corresponds to that range of the reaction force which results from the spread of the nominal value for the pressure drop of the valves of the model used. The spread range around the nominal value is upwardly and downwardly extended in order to also consider deviations of other components, for example, the drives used. However, the valve deviation is here also foremost the determining factor. To build the corresponding threshold values regarding an occlusion and a leakage, the deviation ranges can be increased by tolerance margins. An occlusion is then assumed when the measured reaction force falls upwardly outside the nominal value range predetermined in this way. A leak is assumed if it falls downwardly outside the so-predetermined nominal value range. The valve with defined pressure drop is an advantage, especially for the leak monitoring.
If such a valve is not present or if the pressure drop created by the valve is only so large that the additionally acting forces cannot be neglected in good approximation, several nominal values to be considered for a dispensing operation with the associate upper and lower limits can be preselected. A first nominal value can then be predetermined for that force which must be exerted in order to move the piston in the container forward from a rest position. This force corresponds essentially to that which must be exerted to overcome the contact friction between the piston and the container wall and the flow resistances. Since the speed of advancement during dispensing of the product fluid is generally constant, the piston in the constant phase of its advancement exerts a constant reaction force on its support, the housing, when the catheter is filled. A second nominal value then corresponds essentially to the force which is necessary for overcoming the piston slide friction and the flow resistances which the product fluid encounters on its path from the container to the exit location. Such flow resistances represent, for example, a narrowed outlet of the container, the catheter connected thereto, and an infusion needle connected to a free end of the catheter. An occlusion is preferably then assumed when the measured reaction force exceeds the first nominal value by a preselected maximum amount. A leak is preferably then assumed when the measured reaction force during advancement of the piston is smaller than the second nominal value by a preselected amount.
Nominal values can also be selected in the form of a course of the nominal value over time or the advancement distance which the piston travels during one administration between its position directly before the administration and its position immediately at the end of the administration. The at least one nominal value is in this case a value of a complete nominal value profile or course. Other nominal values of the profile can also be used for comparison purposes.
A priming of the piston or the device can be manually completed or fully automatically carried out. During priming, the piston is advanced from an initial installation position in the container until product fluid exits at the exit location. All fluid conducting parts, essentially the container, the connected catheter and the infusion needle are thereby filled with product fluid after the priming and the first dosed administration can then take place. The completion of the priming can be detected as part of the pressure monitoring in accordance with the invention in that the measured reaction force is monitored if it corresponds or not with the above-mentioned second nominal value after exit of product fluid. When the valve is positioned between the outlet of the container and the needle, this nominal value in good approximation also corresponds to the reaction force generated by the pressure drop between valve inlet and valve outlet.
By comparing the measured reaction force with the at least one nominal value, the fill condition of the container can also be automatically determined. The nominal position of the piston along the axis of displacement of the piston is originally available to the control, since it controls the exact position of the piston drive, preferably a linear drive, relative to a platform. A first contact of the piston drive with the piston can be determined by comparison of the measured reaction force with the at least one nominal value, especially with use of the valve.
In another preferred embodiment, the piston drive is preadjusted by the control in such a way that it stops itself upon first contact against the piston. In this case, the piston position and therefore the fill condition can be determined in an especially simple manner from the known position of the drive motor upon stopping. The use of a self-stopping motor, especially a step motor with a preset or predetermined or adjustable start stop frequency, is principally possible for determination of the fill condition and generally advantageous, i.e., even without the further features of the invention described in connection with the force comparison. The start stop frequency is a maximum frequency, in case of a step motor the maximum pulse frequency, with which the motor starts at a given signal. For a given motor, it is dependent on the momentum to be generated by the motor and is therefore generally presented as a momentum-dependent characteristic frequency line.
A monitoring of the pressure during priming is preferably used for derivation of at least one nominal value for the reaction force, which is specific for each device. The reaction forces during priming itself are compared with the at least one nominal value which is already preset by the manufacturer and, preferably, stored in a non-erasable permanent memory for access by the control.
A reaction force measured during priming is used as an individual nominal value instead of the manufacturer preset nominal value for the pressure monitoring during administration, if the reaction force measured during priming lies within the permissible nominal value range. In that embodiment, the control is adaptive. The sensibility of the pressure monitoring is increased with the adaptive control.
The pressure monitoring in accordance with the invention is most preferably used in portable infusion pump apparatus, as used especially in insulin treatment. In this type of infusion apparatus, the administration of the product fluid, for example, insulin, is almost continuous, in that in short time intervals and over a longer time frame smaller, exactly dosed product doses are administered in the form of many individual administrations. Completely continuous infusions are encompassed by the term administration as are individual injections.